Reduction of noise



April 1938- J D. SCHANTZ 2,113,224

REDUCTION OF NOISE Filed June 13; 1936 s sheets-sheet 1 LIMITING BEFORE LIMI 'I'I/VG rwspuzlvc Y rnzpusnc v N llll Enventor 5 :3 N P Joseph D. e chaniz E S lll 1 y W N m N S (Ittornen April 5, 1938. J. D. SCHANTZ REDUCTION OF NOISE s Sheets-Sheet s I Filed June 13. 1936 3nnntor Schantz (Ittorneu Joseph D Patented Apr. 5, 1933 UNITE smras REDUCTION OF NOISE .loseph D. Schantz, Camden, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 13, 1936, Serial No. 85,041

12 @laims.

This invention relates to the reduction of noise such as that produced in radio receivers and like apparatus by impulses extraneous t0 the signal. It has for its principal object the provision of an improved noise reduction system and method of operation whereby the undesired effects of noise impulses having an amplitude substantially in excess of that of the signal are minimized or eliminated.

Various types of noise reduction systems have been heretofore proposed or used. These systems have generally involved the use of (1) means for balancing or neutralizing the effect of the impulses which tend to produce noise, (2) a remotely located antenna, (3) a limiter or peak clipper, (4) a special transmission circuit, or (5) means for short circuiting, open circuiting or otherwise controlling the transmitting efliciency of the signal channel for the duration of the impulses Which tend to produce objectional noise.

As applied to a signal having a known or determinable amplitude-frequency characteristic, the present invention involves the use of an amplifier designed to produce frequency distortion prior to the limiting action and to restore a desired amplitude-frequency characteristic of the signal subsequent to the limiting action. A copending application of Vernon D. Landon, Serial No. 76,909, filed April 29, 1936 discloses a method of operation which has the advantage that the limiting action may be restricted to a relatively high frequency low amplitude band thus making it possible to limit the noise impulses to a lower level. The present invention is similar in some respects to that of the aforesaid application but diifers therefrom in various details such as the types of amplifiers, theuse of limiting devices instead of channel short circuiting devices, etc.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope is indicated by the appended claims.

Referring to the drawings,

Fig. l is a wiring diagram of a signal channel connected in accordance with the invention,

Figs. 2 to 4 are explanatory diagrams relating to the operation of the system illustrated by Fig. 1,

Fig. 5 is a wiring diagram of a modified noise reduction channel,

Figs. 6 to 8 are explanatory diagrams relating to the operation of the system illustrated by Fig. 5, and

Figs. 9 and 10 are wiring diagrams .of further will have a reactance which is greaterin abso-- lute value than the resistor ll below a frequency f2 (Fig. 2). The resistance of the resistor i9 is made high so that the absolute value of the reactance of the capacitor l8 becomes equal to it at some frequency fl, less than the frequency ft. The eifect of sucha circuit is to produce a tip up of the higher frequencies. The next amplifier tube l5 isolates the tip-up network Il-llll9 and provides additional amplification. The amplifier tube i5 is coupled through means including, a transformer 28 to the succeeding circuits of the channel. 1

Connected across the secondary winding. of the transformer 20 are a pair of diodes 2i and 22 which function to limit the amplitude of the signal transmitted through the channel. These diodes are polarized by means of a battery 23 so that they will draw no current and exert no limiting actionuntil the signal voltage across the transformer secondary exceeds a pr-edeter. mined value. The composite characteristic ofthese diodes and their action in rounding off a peaked wave is shown in Fig. 3 which will be readily understood from the legends applied thereto.

The succeeding network including resistors 23 to 26 and capacitors 2'! and 28 functions to cor rect for the distortion produced by the tip-up network |l'- 'l8l9. Its effect is indicated by the curve of Fig. 4:.

Additional amplification is then supplied by means of stages 29 and 3t and the signal is fed to a loud speaker 3| or other utilization device. v

The system of Fig; 1 is suitable for noise suppression in cases where the noise is of predominantly high frequency. This is the case, for example, of the noiseencountered in sound recordings on film. The amplitude at which the limiting action occurs can be controlled by variation of the polarization voltage on the diodes 2i and 22. Similarly the tip-up of the amplitude-frequency characteristic can be controlled by variation of the resistances and capacities discussed above.

This scheme for noise suppression by limiting action upon a signal having its'amplitude-frequency characteristic altered by amplifier compensation can be made more general by an extension of the preceding idea. Thus assuming that the amplitude-frequency characteristic is indicated by the curve of Fig. 6, any amplitude greater than that indicated by the curve will be undesired noise. In such a case it is therefore desirable to apply limitingaction to all voltages in excess of those which lie on the curve. This result may be accomplished by the system of Fig. 5.

The system of Fig. includes between the stages l4 and I5 parallel resonant circuits 32 to 34 and 35 to 31 and resistors 38 and 39. This network functions to give the amplifier a response characteristic similar to that indicated by Fig. 7. The circuit 3233-34 should be tuned to a low frequency for tipping-up the low frequency components and the circuit 353E3'I should be tuned to a high frequency for tipping up the high frequencies. By adjusting the resonant frequency of each circuit, the type of compensation and the nature of the voltage-frequency characteristiecan be varied more or less at will. The use of the variable resistors 33 and 36 in the circuits renders possible additional control of the characteristic.

The signal is amplified by the amplifier l5 and is then subjected to the limiting action of the diodes 2| and 22 as previously described.

Subsequent to the limiting action, the signal is passed through a network including series resonant circuits 40 to 43 and resistors 44 and 45. This network functions in a well known manner to restore the original amplitude-frequency characteristic as indicated in Fig. 6. Here again (1) the circuits 4!! and 42 are preferably tuned to a low frequency while the circuits 4| and 43 are tuned to a high frequency, (2) the variable capacitors and resistors permit control of the compensation over a wide range, (3) the voltage at which the limitingaction begins is controlled by adjusting the polarizing voltage of the diodes 2i and 22 and (4) the signal is additionally amplified in the stages 29 and 30.

If a signal having the amplitude-frequency characteristic of Fig. 6 is passed through an amplifier with a response characteristic like that indicated by Fig. '7, the amplitude-frequency relation of the signal at the amplifier output obviously will be flat as indicated by Fig. 8. subjecting this signal to limiting action results in limiting uniformly over the entire frequency spectrum. For example, if the amplitude of the desired signal is three volts at this point in the amplifier, then the limiter is so adjusted that no signal of an amplitude greater than three volts is supplied to the succeeding network. Limiting action is thus effected in the optimum manner for minimizing noise. Noise voltages of smaller amplitude than the signal are of course not affected but all noise voltages in excess of the desired signal voltage are suppressed regardless of their frequencies.

The system of Fig. 9 has the advantage that it obviates the necessity of providing push-pull stages beyond the limiter. This system includes a network 46 to 52 between the stages l5 and 29 which is similar to those illustrated by Fig. 5 in the same location. The circuits preceding the' amplifier stage l5 are similar to those of Fig. 5. When the alternating voltage across the secondary of the transformer 20 exceeds the polarizing voltage of the diodes 2| and 22, they draw current thus loading the plate circuit of the amplifier I5 and preventing larger voltage swings. The result is the same as that pointed out in connection with Fig. 1.

Fig. illustrates a system which functions in a manner similar to the system of Fig. 9. It includes a tip-up network 53 to 58 between the stages 14 and I5 and a restoring network 59 to 64 between the stages 2! and 29. The operation of this system will be apparent from the foregoing explanations.

It should be noted that types of rectifiers other than the diodes 2| and 22 may be utilized to obtain the limiting action. t has been assumed that the amplifier stages to which no compensating network has been applied have a substantially fiat response characteristic. This is entirely within the bounds of possibility for a well designed audio amplifier. The systems herein described may be applied equally well to the audio amplifier of a radio receiver or to amplifiers of sound reproducing or reenforcing systems. Use of the various systems illustrated indicates that the system of Fig. 5 is perhaps most effective in the reduction of noise. The use of push-pull stages to provide the limiting action was found to be distinctly advantageous for the reason that direct current is eliminated from the transformer and even harmonics are balanced out.

I claim as my invention:

1. In a signal amplifier, the combination of a signal distorting network, means for amplifying the distorted signal, a pair of polarized diodes for limiting the amplitude of said. signal, a signal restoring network, and means for amplifying said restored signal.

2. In a signal amplifier, the combination of a signal distorting network including a capacitor shunted by a resistor, means for amplifying the distorted signal, means for limitingthe amplitude of said signal, a signal restoring network, and means for amplifying said restored signal.

3. In a signal amplifier, the combination of a signal distorting network including resonant circuits tuned respectively to relatively high and to relatively low frequency components of the signal, means for amplifying the distorted signal, means for limiting the amplitude of said signal, a signal restoring network, and means for amplifying said" restored signal.

4. In a signal amplifier, the combination of a signal distorting network, means for amplifying the distorted signal, means for limiting the amplitude of said signal, a signal restoring network including resonant circuits tuned respectively to relatively high and to relatively low frequency components of the signal, and means for amplifying said restored signal.

5. In a signal amplifier, the combination of a signal distorting network including resonant circuits tuned respectively to relatively high and to relatively low frequency components of the signal, means for amplifying the distorted signal, means for limiting the amplitude of said signal, a signal restoring network including resonant circuits tuned respectively to relatively high and to relatively low frequency components of the signal, and means for amplifying said restored signal.

6. In a signal amplifier, the combination of a signal distorting network, means for amplifying the distorted signal, a pair of polarized diodes coupled to an output circuit of said amplifier for limiting the amplitude of said signal, a signal restoring network, and means for amplifying said restored signal.

7. In a signal amplifier, the combination of a signal distorting network, means for amplifying the distorted signal, a pair of unidirectional conducting devices arranged to operate on alternate half cycles for limiting the amplitude of said signal, and means for imparting a desired amplitude-frequency characteristic to said limited signal.

8. The noise reduction method for a signal amplifying system, which includes distorting the signal to produce a substantially flat amplitude-frequency characteristic, suppressing signal components of an amplitude exceeding a predetermined value, and imparting a desired amplitude-fie quency characteristic to said suppressed signal.

9. In a signal amplifier, the combination of an amplifier coupling network having a response characteristic for accentuating a predetermined range of signal frequencies, amplifying means following said accentuating means, signal amplitude limiting means coupled to said last named amplifying means and including a pair of polarized diode rectifier devices, a second amplifier coupling network following said limiter means having a response characteristic for attenuating said predetermined range of signal frequencies, whereby the response characteristic of the amplifier is restored, and. means for further amplifying the restored signal.

10. In a signal amplifying system, the combination of means for amplifying a signal having a predetermined amplitude-frequency characteristic, means providing an inter-stage coupling network for imparting a substantially fiat amplitude-frequency characteristic to said signal, means following said network for limiting said signal to a predetermined amplitude, whereby the signal is limited uniformly over the full frequency range thereof, and a second inter-stage coupling network following said limiter means to restore the original amplitude-frequency characteristic of the signal.

11. In a signal amplifier, the combination of a signal distorting network including means for increasing the amplitude-frequency characteristic of the signal in a limited range adjacent at least one end of the frequency response range of the amplifier, means for amplifying the distorted signal, means coupled to said amplifying means for limiting the amplitude of said signal and including a pair of diode rectifier devices providing a shunt load on said amplifying means, a signal restoring network following saidlimiter means and including means for attenuating signals in said limited range, and means for further amplifying the restored signal.

12. The combination with a multi-stage signal amplifier, of an inter-stage coupling network providing means for accentuating the amplitudefrequency characteristic of signals transmitted therethrough in a limited range, means coupled with a succeeding amplifier stage for limiting the amplitude of the signal, and a second inter-stage coupling network following said limiter means providing signal attenuating means in said limited frequency range for restoring the response characteristic of the signal transmitted through said amplifier.

JOSEPH D. SCHANTZ. 

